1. Technical Field
The present invention relates generally to band-pass filters and, more particularly, to a monolithically integrated active band-pass filter with a tuneable negative resistance circuit to compensate for resistive losses.
2. Discussion
Narrow band-pass filters which realize low insertion-loss and good shape factors (Q-factor) are required for many commercial applications. With the rapid technological advancements being made in the communications industry, it is important to provide for small, low power, high frequency circuit components for use in wireless hand held portable communications systems. Such applications encompass transmitters and receivers for use with global positioning systems (GPS), direct broadcast systems (DBS), cellular phone systems, local area networks (LAN), wireless internet applications and various other applications. In order to reduce the physical size and power consumption of circuit components, it is often desirable to integrate the filter components onto a single semiconductor chip, when possible. However, reduced size of the integrated circuits often results in a reduced Q-factor which in turn degrades the signal-to-noise ratio and results in poor frequency selectivity of the filter.
The passive circuit components which generally make up a band-pass filter structure are commonly fabricated on a semi-insulating substrate such as a gallium arsenide (GaAs) substrate or an indium phosphide (InP) substrate. Band-pass filter circuit components typically include spiral inductors, capacitors and resistors, which are generally printed, etched, or otherwise fabricated on the semi-insulating substrate. Since semi-insulating materials are not perfect insulators, the quality factor of spiral inductors, capacitors and other passive circuit components formed thereon is limited. Furthermore, resistive losses are commonly present and such losses adversely effect the performance of the filter. In addition, the metal interconnects used in many GaAs and InP semiconductor devices have finite resistances, which further adds to the overall resistive losses. Uncompensated resistive losses tend to degrade the overall quality factor of the filter and add to the overall insertion losses that remain present.
In order to compensate for these resistive losses, a number of approaches have considered the use of an active element to generate a negative resistance feedback signal that is applied to the passive filter structure. In the past, conventional approaches have employed a fixed negative resistance circuit for producing a negative resistance that is applied to the passive filter structure to compensate for resistive losses. For example, in some conventional approaches, a fixed amount of negative resistance is provided, despite the possibility of subsequent changes in the overall resistive losses. Changes in resistive losses may occur, especially due to temperature changes and aging of the filter. Accordingly, many conventional filters offer a very limited tuning capability, if any.
It is therefore desirable to provide for an improved active band-pass filter which provides a negative resistance signal to a passive band-pass filter structure to compensate for resistive losses.
It is further desirable to provide for such a band-pass filter which can be accurately adjusted to cancel out resistive losses and achieve good quality factors and near zero insertion loss.
Yet, it is also desirable to provide for an active band-pass filter with a negative resistance circuit that allows for enhanced Q-factor tunability and offers subsequent tuning capability.
It is also desirable to provide for an active tunable band-pass filter which monolithically integrates circuit components onto a single chip to allow for minimum size requirements and realizes low power consumption.